1. Technical Field
The present invention relates to a production method for water-absorbing resin powder, and in more detail, the present invention relates to a production method for obtaining water-absorbing resin powder having high liquid permeability under highly pressurized condition, by performing surface cross-linking.
2. Background Art
The water-absorbing resin (Super Absorbent Polymer; SAP) is a water-swelling and water-insoluble polymer gelling agent, and is widely used mainly in disposable applications as absorbent articles such as disposable diapers, sanitary napkins; water-retention agent for agriculture and gardening; water-stops for industrial use; and the like. As a raw material of such a water-absorbing resin, many monomers or hydrophilic polymers have been proposed. In particular, a polyacrylic acid (salt)-type water-absorbing resin using acrylic acid and/or a salt thereof as a monomer, is produced most industrially from the viewpoint of high water absorbing performance thereof.
Such a water-absorbing resin is produced via a polymerization step, a drying step, (a removing step of a non-dried substance, as needed), a pulverization step, a classification step, a surface cross-linking step and the like (PATENT LITERATURES 1 to 5). With shift to higher performance of disposable diapers, which are main applications, many functions have been required also for the water-absorbing resin. Specifically, not only simple high absorption capacity but also many properties have been required for the water-absorbing resin, such as gel strength, water-soluble amount, water-absorbing speed, absorbency against pressure, liquid permeability, particle size distribution, urine resistance, antibacterial, impact resistance, powder fluidity, deodorant, coloring resistance, and low powder dust. Therefore, many proposals have been made, such as many surface cross-linking technologies, additives, and changes of production steps, in literatures other than the above or the following PATENT LITERATURES 1 to 49.
Among the above properties, in recent years, liquid permeability has been seen as a more important factor with increase in use amount (for example, 50% by weight or more) of the water-absorbing resin in the disposable diapers. And, many improvement methods or modified technologies of liquid permeability under load or liquid permeability without load have been proposed, such as SFC (Saline Flow Conductivity/PATENT LITERATURE 6) or GBP (Gel Bed Permeability/PATENT LITERATURES 7 to 9).
In addition, in such properties described in the above, many combinations of a plurality of parameters including liquid permeability have also been proposed, and there have been known technology for specifying impact resistance (FI) (PATENT LITERATURE 10), technology for specifying water-absorbing speed (FSR/Vortex) and the like (PATENT LITERATURE 11), and technology for specifying a product of liquid diffusion performance (SFC) and core absorbing amount after 60 minutes (DA60) (PATENT LITERATURE 12).
Further, as a method for enhancing liquid permeability such as SFC or GBP, there have been known technology for adding gypsum before polymerization or during polymerization (PATENT LITERATURE 13), technology for adding a spacer (PATENT LITERATURE 14), technology for using 5 to 17 mole/kg of a nitrogen-containing polymer having a nitrogen atom capable of protonation (PATENT LITERATURE 15), technology for using polyamine and a polyvalent metal ion or a polyvalent anion (PATENT LITERATURE 16), technology for coating the water-absorbing resin having a pH below 6 with polyamine (PATENT LITERATURE 17), and technology for using poly ammonium carbonate (PATENT LITERATURE 18). Other than these, there have also been known technology for using polyamine in a soluble amount of 3% or more, and technology for specifying wicking index (WI) or gel strength (PATENT LITERATURES 19 to 21). In addition, there has also been known technology for using a polyvalent metal salt, as well as controlling a methoxy phenol, which is a polymerization inhibitor in polymerization, to improve coloring and liquid permeability (PATENT LITERATURES 22 and 23). Still more, technology for controlling a bulk specific gravity at a high level by grinding the particles (PATENT LITERATURE 24) has also been known.
In addition, in addition to liquid permeability, water-absorbing speed is also an important fundamental property of the water-absorbing resin, and as a method for enhancing such water-absorbing speed, technology for enhancing water-absorbing speed by enhancing specific surface area has been known. Specifically, there have been proposed technology for controlling a particle diameter finely (PATENT LITERATURE 25), technology for granulating fine particles with large surface area (PATENT LITERATURES 26 to 28), technology for making a porous substance by freeze drying hydrogel (PATENT LITERATURE 29), technology for surface cross-linking of particles at the same time as granulation (PATENT LITERATURES 30 to 32), technology for performing foaming polymerization (PATENT LITERATURES 33 to 48), and technology for foaming and cross-linking after polymerization (PATENT LITERATURE 49) and the like.
In the above foaming polymerization, as a foaming agent to be used in a monomer, specifically there have been known technology for using a carbonate salt (PATENT LITERATURES 33 to 40), technology for using an organic solvent (PATENT LITERATURES 41, and 42), technology for using an inert gas (PATENT LITERATURES 43 to 45), technology for using an azo compound (PATENT LITERATURES 46 and 47), and technology for using an insoluble inorganic powder (PATENT LITERATURES 48) and the like.